"Huh?" Bruce was too concerned about the oncoming HYDRA agents and wasn't sure if Natalia had actually told him to kiss her or if he'd heard wrong. When she started to get more cozy with him, he then realized what she had in mind: he might not have been a super spy like her, but he understood what she wanted to do. The HYDRA agents had been talking to one another and hadn't even noticed them at first, so Bruce smiled wide as though he'd been shamelessly flirting with her.
Kiss me now Boy Natalia wasn't kidding.
Bruce just might've gotten into it a little more enthusiastically than he should: he caressed her cheek with his hand and moved her up against the wall. Their ruse worked, at least it seemed that way - the agents walked right by, only giving them a glance as they passed. Bruce continued to kiss her, not really paying attention that the men had gone down the corridor and around the corner, out of sight.